I propose to study the various control elements that govern regulation of the histidine biosynthetic enzymes in the yeast Saccharomyces cerevisiae. My plan is to analyze the problem from both a genetic and biochemical viewpoint. A series of different selection procedures are outlined which should yield a whole array of different classes of regulatory mutations: mutations which prevent or cause derepression; mutations involved in a general control system for many amino acid biosynthetic pathways, and mutations specific for histidine regulation; mutations which are linked or unlinked to the structural genes they control; mutations at DNA sites and in structural genes; mutations in positive and negative regulatory elements; and point mutations and deletions of the regulatory region adjacent to his4. All of these mutations will be thoroughly analyzed for their genetic properties and regulatory functions. Interactions among the different elements and sites will be studied. As further aids in deciphering eukaryotic gene regulation I propose experiments (1) Which should generate an insertion element capable of integrating the his4 region into different parts of the genome, and (2) Which should permit the identification of genes adjacent to his4 on chromosome III. I also plan to isolate the specific his4 DNA and his4 mRNA as prerequisites for setting up an in vitro transcription-translation system for studying gene regulation.